An internal combustion (IC) engine may include an exhaust gas recirculation (EGR) system for controlling the generation of undesirable pollutant gases and particulate matter in the operation of IC engines. EGR systems primarily recirculate the exhaust gas by-products into the intake air supply of the IC engine. The exhaust gas which is reintroduced to the engine cylinder reduces the concentration of oxygen therein, which in turn lowers the maximum combustion temperature within the cylinder and slows the chemical reaction of the combustion process, decreasing the formation of nitrous oxides (NOx). Furthermore, the exhaust gases typically contain unburned hydrocarbons which are burned on reintroduction into the engine cylinder, which further reduces the emission of exhaust gas by-products which would be emitted as undesirable pollutants from the IC engine.
An IC engine may also include one or more turbochargers for compressing a fluid which is supplied to one or more combustion chambers within corresponding combustion cylinders. Each turbocharger typically includes a turbine driven by exhaust gases of the engine and a compressor which is driven by the turbine. The compressor receives the fluid to be compressed and supplies the fluid to the combustion chambers. The fluid which is compressed by the compressor may be in the form of combustion air or a fuel and air mixture.
When utilizing EGR in a turbocharged diesel engine, the exhaust gas to be recirculated is preferably removed upstream of the exhaust gas driven turbine associated with the turbocharger. In many EGR applications, the exhaust gas is diverted by a poppet-type EGR valve directly from the exhaust manifold. The percentage of the total exhaust flow which is diverted for introduction into the intake manifold of an internal combustion engine is known as the EGR rate of the engine.
With an EGR system using an EGR valve as described above, it is desirable to recirculate an amount of exhaust gas within a relatively small tolerance range around a target EGR rate. Venturis are widely used as flow meters on competitive diesel engines to measure exhaust gas flow recirculated to the intake manifold. Venturis are useful because a pressure differential exists across the device which can be correlated to a mass flow rate.
A problem with conventional venturis used in an EGR system is that diesel combustion products build up in the venturi and affect the internal geometry of the venturi, in turn affecting the differential pressure measurement across the venturi. Accurate measurement of EGR is essential to controlling the emissions of the engine.
Another problem with conventional venturis used in an EGR system is that the sensor for flow measurement is sensitive to temperature both for the life of the sensor (overheating) and repeatability of the sensor (temperature changes). In normal practice the sensor must be protected from the heat of combustion products by either remote mounting or shielding to prevent damage. On the other hand, ambient temperature variation reduces the repeatability of the sensor.
What is needed in the art is a venturi for an EGR system which is not subject to overheating and has a higher reliability.